1. Field of the Description
The present description relates, in general, to robotic joints, and, more particularly, to a robotic joint using linear actuators in combination with four-bar linkages to drive side gears of a differential to provide a 3-axis joint that replicates movements of a human or human-like shoulder joint with a similar form factor, e.g., the robotic joint and linear electrical actuators can be mounted within a structure or shell having human or similar dimensions and/or form.
2. Relevant Background
There are many applications for robotic joints. Many characters or figures including those found in theme parks are animated with limbs that move using robotic joints. Effective animated figures, e.g., animatronic figures that are human or human-like such as characters given human qualities and movements, have been created using robotics. However, it has proven difficult to design a robotic joint that can effectively simulate human joints and, particularly, human shoulders, hips, elbows, and similar joints. For example, difficulties with designing robotic shoulder joints include the relatively tight or small form factor provided by the figure's structure at or near the shoulder. For example, a character with human or similar proportions would need to contain all the components of the shoulder joint and internal machinery within the skin or covering over the shoulder or nearby such as in the body cavity (e.g., within the form of the human body or the character body). Another shoulder design challenge involves providing the range of motion provided by human and other similar shoulder joints at the same speed and providing arms or limbs with desired strength, e.g., similar or greater than a human.
Early industrial robotics used hydraulic actuators. While appearing in general shape and function to be “arms” that rotate about a shoulder joint, most designs had no form factor constraints similar to a human body's external envelope constraint. As a result, hydraulic actuator-based robotics designed for industrial use generally do not lend themselves to use with joints simulating human joints or representing a human shape or its shoulder function. Hydraulic actuators include a hydraulic power supply made up of an electric pump, an oil tank, filters, accumulators, and associated components. The power supply is used to create a high pressure source of hydraulic fluid that is piped to a manifold that houses a series of hydraulic servo valves, which meter oil to hydraulic cylinders placed local to each joint or axis of motion of a robot or animated figure. A control computer may be used to provide commands or control signals to the various servo valves to achieve a desired movement of the hydraulically actuated joint.
Hydraulically actuated robotic joints have a number of advantages including the high power density (e.g., high power or force for a given size) of hydraulic cylinders. Also, these joints are relatively easy to design and use in part because they may be attached simply by using spherical rod ends that make it easy to create pivoting joints. These robotic joints also have long lives since the contacting elements include sliding seals that are intrinsically oil lubricated. Hydraulically actuated robotic joints have many offsetting disadvantages including the fact that hydraulic systems are typically messy and dirty as they leak oil that attracts dirt and stains the animatronic figure including the joints, skins, clothing, and/or other figure finishing. Oil also causes skin materials to break down and can be a fire hazard. These joints may be dangerous to operate due to the high pressure oil used for power that potentially can spray out of holes in joints and hoses injuring passersby (e.g., guests of a theme park, maintenance personnel, and others nearby to the animatronic figures). Use of hydraulic actuators requires the use of a hydraulic power unit that may be noisy and require pumps, tanks, filters, piping, and cooling mechanisms. It is often hard to run the needed and numerous hydraulic lines through and around the joints due to limited flexibility and size of tubing that can handle the high operating pressures. Also, the achievable servo bandwidth is limited by the compressibility associated with the length of hydraulic lines between the servo valves and the hydraulic cylinders, and further, the servo valves are too large to fit within the external envelope or to conform to a desired form factor of a human or other animatronic figure. Additionally, it is difficult to make such figures mobile or portable due to the size and noise associated with the hydraulic infrastructure.
Due to these limitations, electric motors have been used for at least the past twenty years in place of hydraulic actuators in commercial robotics. However, a number of problems have made it difficult to design a proper form factor robotic shoulder joint. In electric actuators, electronic amplifiers are commanded to supply specified currents to electric motors. The motor is typically placed local or in the joint of the robot or animated figure. As with hydraulic actuators, the commands or control signals provided to the amplifiers are generally provided by one or more controllers or control computers. Electric actuators have the advantage over hydraulic actuators of being clean and easy to maintain. Also, the behavior of electric motors is well understood and is useful for creating repeatable and controllable motions. With electric actuators, it is relatively easy to monitor force output using motor currents, which is helpful in certain control tasks and allows use of simple methods to limit output force to ensure safety.
Unfortunately, electric actuators typically have lower power density when compared with hydraulic actuators making it difficult to achieve desired accelerations. To achieve high power, electric motors must operate at high speed, and this requires a speed reduction mechanism. As a result, electric actuators often require complicated mechanical designs or configurations because of the speed reduction required between the motor and the joint and due to the form factor of the electric motor. Other joint designs have driven the differential with cables or gears but have placed the drive motors or actuators in or near the joint, which makes compliance with the form factor difficult and also undesirably increases moving inertia as the motor mass moves along with the joint components. This, in turn, reduces achievable accelerations or motion performance and can also reduce load carrying capacities.
Some efforts have been made to address form factor using a cable-type transmission to drive differential side gears in a robotic joint, but such designs are often difficult to design to provide a very high cycle life, which is required for many applications such as theme parks and other entertainment and industrial applications. In other cases, geared differentials are used to drive robotic joints but use bevel gears, which does not address human form-factor packaging constraints (e.g., is typically only suited to industrial applications with little or no size/packing constraints). Hence, electric actuators have not proven widely useful for creating shoulder and other “human” joints due to difficulty in complying with the associated form factors and due to power and speed constraints. Further, existing humanoid robots may have a human-like appearance but do not meet human form factor requirements and may also differ from humans in their shape, range of motion performance, and/or degrees-of-freedom.
There remains a need for an improved mechanism for use as a shoulder joint in robots or animatronic figures or characters. Preferably, the mechanism would simulate the movement and functionality of a human shoulder joint and would be configured to comply with the form factor of a human shoulder or human-like proportions for a shoulder (e.g., within the robotic figure's shoulder and body cavity or structure). Additionally, it is preferable that the shoulder mechanism be safe and clean to operate.